I would fold the covering over the trailing adge and seal it down with the iron across the entire scallop, this will assure the covering will remain in the bottom of the scallop like it is supposed to be, you may have to slit the covering every 1/2" or so to get it to conform good. The full scale actually had a wire trailing edge and the scallop was formed by fabric being shrunk and the wire being pulled in. A little late but Sig Koverall would be perfect for this job.
Anthony